All disks DEAD - Everything works .... ReadyNAS Ultra 6 Plus

Yes, I meant OS4.2, not OD.  That's the firmware family for legacy Intel NASes.  I switched to OS6 well before the version you are on.  I noticed the drive speed difference from the screen capture you provided, which is why I offered my experience of something perhaps similar. 

Yes, you should remove and replace the drives one at a time with power on.  Give the NAS time to report the removal before re-inserting.  Each will need to re-sync, so total time is going to be a lot.

Actually, you can switch to OS6 on an Ultra6.  You have to re-format the drives and you lose any Netgear support (including paid support), but not forum support.  There are a lot of positives about switching to OS6.  It's more modern and kept up to date and has fewer expansion limitations.  A big one is if your NAS dies, or you just want to upgrade to something faster, you can move the drives right into a new OS6 based NAS and just keep trucking.  No having to transfer the data because of the different file system (you already had to do that to upgrade to OS6).

It's a good idea to bump the memory up to at least 2GB before switching to OS6, but memory for an Ultra 6 is quite inexpensive these days.